In the most conventional backup systems, the amount of information that will be written by a backup operation is unknown at the time that the backup operation begins. Accordingly, it is possible to select a destination storage device for the backup that ultimately does not have enough free storage space to store the entire backup. For example, assume the total amount of backup information is 120 GB and the free storage space of the selected storage device is only 100 GB. When the backup operation attempts to transfer information in excess of 100 GB to the selected storage device, the selected storage device will not be able to store the additional information. This in turn causes the backup operation to fail due to the storage overflow.
A storage overflow significantly delays completion of the backup, since a new backup operation targeting a different storage device will need to be started, and any information that was already written to backup by the failed backup operation will need to be transferred again to the new storage device. As this example shows, it is desirable to be able to increase backup efficiency in situations in which the originally-selected target storage device is not able to store all of the backup information.